286 Plant Physiology 



many physiologists respiration has long been considered 

 to be a combustion process. If by combustion one under- 

 stands a direct union of the O 2 with C in such a manner 

 that C0 2 is a direct result, this comparison is unfortunate. 

 The combustion of any product results in a perfectly 

 definite amount of energy as heat ; and this heat-energy 

 may be very simply determined, whether it involves the 

 combustion of coal, of proteins, of starch, or of cane- 

 sugar. There is, furthermore, in combustion (however 

 produced) a definite relation between the amount of oxy- 

 gen needed and the amount of carbon dioxid given off. 

 There is therefore a definite CO 2 / O 2 ratio ; thus the 

 combustion of glucose would require 6 molecules of O 2 , 

 and 6 molecules of CO 2 would be produced, by the follow- 

 ing formula : - 



c JT o 6 + 60=6 co;+ 6 HA 



The combustion quotient in this case is unity. In respi- 

 ration the transformations are not necessarily complete, 

 and the respiratory quotient is seldom exactly unity, and 

 as a consequence there are by-products and products 

 less stable then CO 2 . The quotient is affected by tem- 

 perature and other environmental conditions ; thus it is 

 possible to picture a more complex and less definite se- 

 quence of changes in which protoplasm is involved. The 

 series of transformations may be of the same type in the 

 two cases, but they are not properly regarded as compa- 

 rable processes so far as may be determined at present. 

 166. Respiratory activity. - - Respiratory activity is 

 greatest during periods of rapid growth and differentiation. 

 So soon as adequate water is absorbed by seeds previously 



